Not applicable.
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(1) Field of the Invention
The present invention generally relates to high strength stainless steels suitable for forming rotating turbine components. More particularly, this invention relates to a precipitation-hardened stainless steel containing both carbon and niobium at a specified ratio, a low nitrogen content, and processed to have a grain size and at certain tempering temperatures to provide an excellent combination of strength and toughness.
(2) Description of the Related Art
Specific strength and toughness requirements must be met for rotating steam turbine components in order to achieve safe and efficient operation. For more demanding turbine environments, conventional stainless steels in current use do not achieve a desired combination of mechanical properties at a cost that permits their widespread use. For example, conventional stainless steels having ultimate tensile strengths (UTS) of 175 ksi (about 1200 MPa) or more generally do not exhibit a Charpy impact toughness of more than 40 ftxc2x7lbs (about 55 J).
U.S. Pat. No. 3,574,601 discloses the stainless steel commercially known as Carpenter Custom 450, and focuses on corrosion resistance in the overaged condition. A maximum UTS of 152.5 ksi (about 1050 MPa) is reported for the alloy in the patent. The literature regarding this alloy reports an aging temperature range of 800xc2x0 F. to 1000xc2x0 F. (about 427xc2x0 C. to 538xc2x0 C.), with aging at 900xc2x0 F. (about 480xc2x0 C.) producing maximum strength but lowest toughness. The literature also reports a UTS of greater than 175 ksi (1200 MPa) after aging at 900xc2x0 F. to 950xc2x0 F. (about 480xc2x0 C. to 510xc2x0 C.), but an Izod impact toughness of less than 70 J. The Custom 450 alloy contains chromium, nickel, molybdenum and copper, as well as other potential alloying constituents such as carbon and niobium (columbium), to yield essentially a martensitic microstructure. To stabilize carbon as an austenite former, niobium may be added at a weight ratio of up to ten relative to carbon if carbon is present in an amount above 0.03 weight percent. U.S. Pat. No. 3,574,601 teaches that heat treatment is not critical for the Custom 450 alloy, and any effect that grain size might have on the properties of the alloy is not discussed.
The literature has reported on the significant role that nitrogen and carbon have on the impact toughness of martensitic stainless steels containing 12 weight percent chromium. The negative effects of the delta (xcex4) ferrite phase on mechanical properties are also well documented for 12% Cr martensitic stainless steels. Finally, grain size is known to effect impact strength, based in part on the increase in cleavage fracture stress corresponding to decreasing grain size. Notwithstanding the above, a martensitic stainless steel that exhibits the combination of high strength and toughness is not available. Instead, efforts have largely been directed to martensitic stainless steels that emphasize increased corrosion resistance over strength for use in highly corrosive environments, such as the alloys disclosed in EP0649915A1 and EP0384317A1 and intended for the transportation of oil or natural gas.
In view of the above, it would be advantageous if a stainless steel existed that offered a unique combination of high strength and high toughness suitable for rotating steam turbine components.
The present invention provides an improved stainless steel alloy that exhibits both high strength and toughness as a result of having particular ranges for chemistry, tempering temperatures and grain size. The alloy of this invention is a precipitation-hardened stainless steel, in which the hardening phase includes copper-rich precipitates in a martensitic microstructure. Required mechanical properties of the alloy include an ultimate tensile strength (UTS) of at least 175 ksi (about 1200 MPa), and a Charpy impact toughness of greater than 40 ftxc2x7lb (about 55 J). According to the invention, these properties are obtained with a precipitation-hardened stainless steel alloy consisting essentially of, by weight, 14.0 to 16.0 percent chromium, 6.0 to 7.0 percent nickel, 1.25 to 1.75 percent copper, 0.5 to 1.0 percent molybdenum, 0.03 to 0.5 percent carbon, niobium in an amount by weight of ten to twenty times greater than carbon, the balance iron, minor alloying constituents and impurities. Maximum levels for the minor alloying constituents and impurities are, by weight, 1.0 percent manganese, 1.0 percent silicon, 0.1 percent vanadium, 0.1 percent tin, 0.030 percent nitrogen, 0.020 percent phosphorus, 0.025 percent aluminum, 0.008 percent sulfur, 0.005 percent silver, and 0.005 percent lead.
The above alloy differs from the Carpenter Custom 450 stainless steel in several important aspects. First, the focus of the Custom 450 alloy is corrosion resistance, and not a combination of high strength and toughness. To achieve improved toughness as compared to Custom 450, the alloy of this invention relies on a particular combination of chemistry, microstructure and tempering temperature. In terms of chemistry, the alloy employs a very narrow range for carbon content, a range of Nb/C ratios higher than Custom 450 (U.S. Pat. No. 3,574,601, and a very limited nitrogen content to promote impact toughness. Also preferred to meet the required impact toughness is a grain size of ASTM 5 (average grain diameter of 62 micrometers) or finer, and more preferably a grain size of ASTM 7 (average grain diameter of 32 micrometers) or finer. Also important to the alloy is a delta ferrite content of less than 0.5 weight percent, in view of the negative effects of the delta ferrite phase on mechanical properties. Finally, the processing of the alloy includes an austenizing heat treatment at a temperature of about 980xc2x0 C. to about 1100xc2x0 C., followed by tempering (aging) at a temperature of about 900xc2x0 F. to 975xc2x0 F. (about 480xc2x0 C. to about 525xc2x0 C.). The tempering heat treatment is particularly important to obtaining the strength and impact toughness properties required by this invention. Aside from the assumption that the alloy does not contain any prior melt or process related defects, it is believed that the desired mechanical properties of the precipitation-hardened stainless steels of this invention do not rely on prior melting practices, thermal mechanical processing, and heat treatments as long as the chemistry, grain size and tempering range (480xc2x0 C. to about 525xc2x0 C.) conform to that noted above.
In view of the above, those skilled in the art will appreciate that the precipitation-hardened stainless steel alloy of this invention is characterized by a combination of both high tensile strength and impact toughness at levels that exceed the capability of commercially available stainless steel alloys with similar chemistries. The alloy achieves these advantages without special processing techniques, other than the tempering temperature range noted above. As a result, the alloy of this invention is well suited for such demanding applications as rotating components for steam turbines.
Other objects and advantages of this invention will be better appreciated from the following detailed description.